Many years and many websites ago, I had an author contact me to review the book that he claimed he'd just written--all by himself. I had a quick look, and found a whole lot of content lifted directly from my website. No acknowledgements. Just a lot of stolen content.
Apparently, he'd worked hard to develop the content in that book and he was a leader in his field. I emailed him back and gave him my thoughts on copyright. In a panic, he responded with a very quick turnaround. He didn't write that book after all. He'd hired a ghostwriter from a site like upwork, and that ghostwriter had lifted the content from my website. The moral of the story is don't use those sites to look for a writer. I've used them on occasion to solve technical problems with my websites, but I'd never go looking for a writer there.